1. Field of the Invention
The present invention relates to heave compensators for use on a floating platform to tension an oil production riser extending to the platform from the ocean floor.
2. Description of the Prior Art
Production risers from offshore oil wells normally must be supported at the water surface to prevent collapse. This is readily accomplished when the oil production platform is fixed to the ocean floor, but presents a significant problem when the water depth is so great that the platform must be floating and hence is not stationary.
Riser tensioners have been developed for use during drilling operations from a floating platform to compensate for rise and fall of the platform from ocean swells. Such tensionsers have commonly comprised hydraulic compensating cylinders connected by cables to the riser at symmetrically arranged tie points. In the course of their travel from the cylinders to a riser, the cables pass over one or more sheaves and hence are subject to wear. It thus becomes necessary to periodically adjust the cables so that unworn cable is shifted to the sheave locations, and, from time to time, the cables must be replaced. To make this cable replacement and cylinder repair possible, a duplicate riser tensioner system with independent riser connections has normally been installed as a backup.
The described riser tensioners used on floating drilling platforms could be used on a floating production platform, but the cable wear problem experienced therewith is more pronounced because the productive life of the well is so much longer than the original drilling of the well. Furthermore, it is not only necessary to have a backup system so that major repairs can be made, but it is preferred that the backup system automatically take over in case of failure of the normally active system.